Proton affinity transfer reaction

Note The reactions were studied by Wilson et al. in a SIFT at 300 K.93 HCNH+/C2H4 and CHj/CH,CN were produced in the flow tube by reactions 46 and 47a. CjHjNCH and C2HSCNH+ were produced directly from Cj NC and C2H5CN in a high pressure ion source and by proton transfer to these neutrals from HCO+ in the flow tube. Proton affinities (kcal mol-1) are given below each reactant neutral and the proton detachment energies of the ions are below the ion identification.2 30 his indicates not studied. 

Since the proton affinity P(Rn=) of the branched olefin Rn= is certainly greater than P(C2H4), the reaction would be endothermic a similar argument applies to the transfer of carbonium ions. This may account for the low yields observed even in the heterogeneous cationic oligomerisation of ethylene. The most likely transfer reactions are hydride ion and methide ion transfers between oligomer ions and molecules (see below), leading to conjunct polymerisation [12]. 

As Skinner has pointed out [7], there is no evidence for the existence of BFyH20 in the gas phase at ordinary temperatures, and the solid monohydrate of BF3 owes its stability to the lattice energy thus D(BF3 - OH2) must be very small. The calculation of AH2 shows that even if BFyH20 could exist in solution as isolated molecules at low temperatures, reaction (3) would not take place. We conclude therefore that proton transfer to the complex anion cannot occur in this system and that there is probably no true termination except by impurities. The only termination reactions which have been definitely established in cationic polymerisations have been described before [2, 8], and cannot at present be discussed profitably in terms of their energetics. It should be noted, however, that in systems such as styrene-S C/4 the smaller proton affinity of the dead (unsaturated or cyclised) polymer, coupled, with the greater size of the anion and smaller size of the cation may make AHX much less positive so that reaction (2) may then be possible because AG° 0. This would mean that the equilibrium between initiation and termination is in an intermediate position. 

Not all ionization methods rely on such strictly unimolecular conditions as El does. Chemical ionization (Cl, Chap. 7), for example, makes use of reactive collisions between ions generated from a reactant gas and the neutral analyte to achieve its ionization by some bimolecular process such as proton transfer. The question which reactant ion can protonate a given analyte can be answered from gas phase basicity (GB) or proton affinity (PA) data. Furthermore, proton transfer, and thus the relative proton affinities of the reactants, play an important role in many ion-neutral complex-mediated reactions (Chap. 6.12). 

More recent work revealed the importance of gas phase proton transfer reactions. [91-94] This implies that multiply charged peptide ions do not exist as preformed ions in solution, but are generated by gas phase ion-ion reactions (Chap. 11.4.4). The proton exchange is driven by the difference in proton affinities (PA, Chap. 2.11) of the species encountered, e.g., a protonated solvent molecule of low PA will protonate a peptide ion with some basic sites left. Under equilibrium conditions, the process would continue until the peptide ion is saturated with protons, a state that also marks its maximum number of charges. 

In essence, solvent ions can transfer a proton to the analyte if the analyte s proton affinity (PA) is higher than that of the eluent molecules, by means of gas-phase reactions. 

We turn to the chemical behavior of cycloalkane holes. Several classes of reactions were observed for these holes (1) fast irreversible electron-transfer reactions with solutes that have low adiabatic IPs (ionization potentials) and vertical IPs (such as polycyclic aromatic molecules) (2) slow reversible electron-transfer reactions with solutes that have low adiabatic and high vertical IPs (3) fast proton-transfer reactions (4) slow proton-transfer reactions that occur through the formation of metastable complexes and (5) very slow reactions with high-IP, low-PA (proton affinity) solutes. 

Class (3) reactions include proton-transfer reactions of solvent holes in cyclohexane and methylcyclohexane [71,74,75]. The corresponding rate constants are 10-30% of the fastest class (1) reactions. Class (4) reactions include proton-transfer reactions in trans-decalin and cis-trans decalin mixtures [77]. Proton transfer from the decalin hole to aliphatic alcohol results in the formation of a C-centered decalyl radical. The proton affinity of this radical is comparable to that of a single alcohol molecule. However, it is less than the proton affinity of an alcohol dimer. Consequently, a complex of the radical cation and alcohol monomer is relatively stable toward proton transfer when such a complex encounters a second alcohol molecule, the radical cation rapidly deprotonates. Metastable complexes with natural lifetimes between 24 nsec (2-propanol) and 90 nsec (tert-butanol) were observed in liquid cis- and tra 5-decalins at 25°C [77]. The rate of the complexation is one-half of that for class (1) reactions the overall decay rate is limited by slow proton transfer in the 1 1 complex. The rate constant of unimolecular decay is (5-10) x 10 sec for primary alcohols, bimolecular decay via proton transfer to the alcohol dimer prevails. Only for secondary and ternary alcohols is the equilibrium reached sufficiently slowly that it can be observed at 25 °C on a time scale of > 10 nsec. There is a striking similarity between the formation of alcohol complexes with the solvent holes (in decalins) and solvent anions (in sc CO2). 

These species, for oxygen-containing compounds, have a large stability (in excess of 20 kcal mol - ) and require a very loose association. This stability allows for partial proton transfer to the alcohol, even when the ester has a larger proton affinity than the alcohol. The scheme is also consistent with the picture that reactions will take place only at centres prone to form a carbenium ion, namely secondary or tertiary carbon centres. 

The protonation reaction leading to MH + depends on the proton affinity of M. Because hydrocarbons of type RH have a low proton affinity, the R+ ion is often seen in the spectrum. Since proton transfer in chemical ionisation can lead to either M + H + or M — H 1, uncertainties can exist as to the true mass of the molecular ion. 

The quantum-chemical calculation of charge-transfer states as possible intermediates in electrophilic aromatic substitution reactions, making allowance for solvation effects, has been reviewed.6 It has been shown that a simple scaled Hartree-Fock ab initio model describes the ring proton affinity of some polysubstituted benzenes, naphthalenes, biphenylenes, and large alternant aromatics, in agreement with experimental values. The simple additivity rule observed previously in smaller... 

The vertical electron affinity (EA) of acetone is given as —1.51 eV by Jordan and Burrow386. Lifshitz, Wu and Tiernan387 determine—among other compounds—the excitation function and rate constants of the slow proton transfer reactions between acclone-Ih, acetone-Dg and other ketones. The acetone enolate anion has been produced in a CO2 laser induced alkane elimination from alkoxide anions by Brauman and collaborators388-390. These show, e.g. that the methane elimination from t-butoxide anion is a stepwise process ... 

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